import numpy as np
import cv2

#Create a black image
img = np.zeros((512,512,3),np.uint8)

#draw a diagonal blue line with thickness of 3 px
cv2.rectangle(img,(350,0),(500,128),(0,255,0),3)

cv2.namedWindow('image',cv2.WINDOW_NORMAL)
cv2.resizeWindow('image',1000,1000)
cv2.imshow('image',img)
cv2.waitKey(0)
cv2.destroyAllWindows()
